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q ,CUBIC
COMMUNICATIONS
SIMAltI
CIVISION
SOLID STATE SWITCHING
One of the
more useful innovations in recent years
is the application
of solid state
switching to signals within
receivers and transmitters.
It
allows switching to be accomplished at the site,
without the need for rout-
ing long signal leads to a bulky,
complex, and often troublesome multi-pole
mechanical switch.
Instead, a simple switch
controls a
De
voltage which in
turn is applied to all of the points
where switching
of the
signal
is to
take place.
The simplest form
of a solid state
switch is the diode.
We know that
when it is reverse biased,
it presents a very high
resistance-~high
enough
to be considered
an open circuit when
used in low or medium impedance cir-
cuits.
Modern
manufacturing techniques have
reduced the junction capacit-
ance of switching
diodes so low
that they can also
be used effectively in
most
high impedance circuits, too.
Thus,
a diode which has a reverse bias
voltage greater than the peak value of any AC signal applied toit,
so that
it does not conductduring any part of the signal cycle, will act as an open
circuit.
Conversely,
modern diodes have a very low
"on" resistance, so if for-
ward biased
with a current greater than the peak signal current, so that it
conducts during all ·parts
of the
signal cycle,
it passes
the signal with
little or na attenuation.
All
that is
needed,
then,
is a circuit arrangement of capacitors to
block the
De
but pass the signal,
(and perhaps bypasses,
if high-level RF
is near) and resistors to apply the bias and switching voltages, and the di-
ode itself.
Field Effect Transistors (FET's) can be used as even better solid state
switches.
Remembering
that the source to drain
path of the
FET is a pure
resistance, normally conducting,
we can see that,
as with any other resis-
tance, current can flow in
either direction
(bilaterally),
from source to
drain or from drain to source, depending on the polarity of the applied vol-
tage.
Therefore, AC signals will pass.
Now,
if we apply
a voltage
to the gate that is
sufficient to "pinch
off" the source-drain path,
the effective resistance of that path increases
greatly, effectively opening the circuit, and thence the signal path.
Normally,
N-channel, depletion mode devices, with good bilateral char-
acteristics
in the source-drain path
and low "on" resistance
are selected
for service in switching applications,
although
P-channel devices are also
used.
All, then, that is needed for a FET switching circuit is the FET, suit-
able blocking capacitors and load and biasing resistors, just as in the case
of the diode
switch.
In some in stances ,
even these can be omitted,
using
the signal voltages themselves to provide any bias.
TWO TONE TESTING
Probably the most useful and yet least often performed test ofa single
sideband transmitter's performance
a technician
can make
is the two
tone
test,
and yet it is easily performed.
The two tone test
will disclose qu-
ickly and easily most of the major faults that are to be found in an operat-
ing transmitter
by
showing nonlinearities,
improperly adjusted bias, over-
driving, and many other problems.
While a full discussion
of the two tone test and lts results is beyond
the scope
of this manual,
a brief discussion
is in order.
More
thorough
7-)
7-
L...o.----....,....------
...:,......::..J
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